This invention relates to an image-sensing device, and more particularly to a solid state image-sensing device which enables an image signal corresponding to the illumination by an introduced light to be read out with a relatively slow speed.
Recently a picture-photographing apparatus has been developed which can pick up an optical image without applying a sensitized film containing silver. This picture-photographing apparatus is used in substitution for the conventional photographic camera device. The picture-photographing apparatus contains a solid state area image-sensing device, which generates an image electric signal corresponding to the luminosity of an introduced light. After amplified, the image signal is transmitted or transferred to signal-recording means, for example, the ordinary voice tape recorder. The image electric signal is recorded on a magnetic tape of a tape recorder by the known process.
The reason why a voice tape recorder is applied as means for recording an output image signal from the solid state image-sensing device is that the picture-photographing apparatus can be reduced in size, thereby decreasing its power consumption and manufacturing cost.
Since, however, a certain limitation is imposed on the acceleration of the running speed of a magnetic tape of a tape recorder, an image signal can not be recorded on the magnetic tape with a high speed. Consequently, a solid state image-sensing device held in the picture-photographing apparatus is so arranged as to release signal charges generated in accordance with the illumination by an introduced light from a storage section at such a low speed as consumes scores of seconds per discharge. Where signal charges are released from the storage section at the above-mentioned low speed, then the signal charges are retained in the storage section for a longer time. This event leads to an increase in the amount of noise charges, for example, undesired thermally generated dark current charged in the storage section, and consequently the percentage of dark current charges mixed with optically generated signal charges corresponding to an image signal. Dark current charge behaves as noises resulting from, for example, shading, irregularities and scars appearing in a reproduced image. Therefore, an increased content of dark current in image signal charges gives rise to objectionable events such as a decline in the dynamic range of a picture-photographing apparatus and the deterioration of a picture quality.
Dark current charge originates with a different cause from that by which an optically generated signal charge is produced. However, the dark current charge has substantially the same physical properties, (for example, polarity) as the optically generated signal charge, presenting difficulties in separating a dark current charge from an optically generated signal charge. Moreover, the dark current charge intricately changes with various factors such as time, ambient temperature and the site at which the dark current is generated. This fact makes it more difficult to separate the dark current charge from the optically generated signal charge.
To eliminate the above-mentioned difficulties, therefore, cooling means has hitherto been provided in the picture-photographing apparatus in order to restrict or prevent the growth of a dark current charge. Provision of the cooling means indeed attains the object of limiting or suppressing the generation of the dark current charge, but is still accompanied with the drawbacks that a picture-photographing apparatus has a complicated arrangement and increases in size, cost and power consumption.
To resolve the aforesaid difficulties, it may be contemplated to apply the process of providing memory means in the picture-photographing apparatus in addition to an image sensor, transmitting an optically generated signal charge corresponding to an output image signal from the image sensor at high speed to the memory means and later, as need arises, read out the optically generated signal charge from the memory means. In this case, however, it is necessary to enlarge the capacity of the memory means in accordance with a number of unit cells of which the image sensor is formed. Consequently, the above-mentioned process still has the drawbacks that the memory means has an enlarged size and complicated arrangement, increasing the cost of manufacturing a picture-photographing apparatus.